A pressure-difference pickup of such type is disclosed, for example, in Offenlegungsschrift DE 196 08 321 A1. Pressure-difference pickups include a hydraulic body, in which an overload chamber with an overload membrane is constructed. The overload membrane divides the overload chamber into a high-pressure half and a low-pressure half, with the high-pressure half communicating with a first hydraulic path extending between a first pressure intermediary (including a separating membrane, or diaphragm, over a membrane bed) and the high-pressure side of a pressure measuring cell, and the low-pressure half communicating with a second hydraulic path extending between a second pressure intermediary (including a separating membrane, or diaphragm, on a membrane bed) and the low pressure side of the measuring element.
In the case of large overloads, the transfer liquid is pressed completely out of the pressure intermediary, and the separating membrane comes to rest against the membrane bed. The transfer liquid pushed into the relevant hydraulic path in the case of overload pressure leads to a deflection of the overload membrane, in order to accommodate the additional volume and reduce the overload pressure.
The stiffer the overload membrane, the faster the measurement cell reacts to pressure fluctuations. This is especially important in the case of sensors with pressure intermediary appendages connected with the sensor via a long capillary line. The long capillary line has a large hydraulic resistance and forms with the overload membrane an RC-element. A soft overload membrane with a large hydraulic capacitance can lead to large time constants, i.e. long response times. In principle, however, even a short hydraulic measurement path, coupling a pressure intermediary (perhaps itself even integrated in a hydraulic body) with a pressure measurement cell, forms, together with an overload membrane, an RC-element, which can affect the dynamics of the measurement.
Currently used overload membranes exhibit, to a first approximation, a constant capacitance, i.e. a linear characteristic curve. That is, the volume increase dV is proportional to the pressure change dP. In this way, already in the measuring range, the dynamics of the pressure difference pickup are affected by the overload membrane.